Hitherto, blackboards (chalk boards) have a function that characters, pictures or the like are erasably written/drawn on the surface thereof with a chalk, and have been used to assist various explanations, lectures, speeches or the like. Thus, in conventional blackboards (chalk boards) as prescribed in JIS S 6007, the surface thereof has appropriately roughness in order that characters, pictures or the like can easily be written/drawn with a chalk or can easily be erased, and further dark color material has been used for the surface thereof in order that the written/drawn characters, pictures or the like can easily be watched.
In recent years, however, it has been become general that various projectors such as lantern slides or OHPs are used to project materials prepared in advance onto a screen and then an explanation is made while either one of the projected images is pointed with a painter or the like. In the case of making an explanation or the like using various projectors, it is necessary to prepare both of a projecting screen and a blackboard, on which characters or the like can be written/drawn with a chalk and can be erased to make a supplementary explanation and thus, considerably much time is required for the arrangement thereof and tire settlement thereof. The reason therefor is as follows: when images are projected to a conventional blackboard with any one of various projectors, the quality of the projected images are dark and unclear, thus, no conventional blackboard has been used as a screen.
On the other hand, in reflection screens, a main object of which is that images through various projectors is projected thereon, material in white color or a highly bright color is used therefore in order to improve the reflective property. Accordingly, when writing or drawing is made onto such a screen with a chalk (which chalk), a low contrast is given. Thus, the recognizable property of characters, pictures or the like decreases. Ordinary screens such as beads or white mats have a low surface hardness; thus, when writing or drawing is made thereon with a chalk (white chalk) and the resultant is erased, a problem that the screens are worn away is caused. Therefore, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 02-153080 and Japanese Patent Application Laid-Open (JP-A) No. 04-229299, screens are not used as chalk boards although the screens are used as marker boards, on which writing or drawing with a marker and erasing are made.
Against such problems, for example, Japanese Utility Model Application Laid-Open (JP-U) No. 57-183190 suggests an enamel blackboard which is also used as a projecting screen, having a rough-surface enamel layer on its surface which informed on a steel sheet using a glaze containing a blue dye and have a brightness of 4 to 8 on a steel sheet.